Planetary gear systems are known. Examples of planetary gear systems may be found in U.S. Pat. Nos. 6,994,651 and 7,297,086 and U.S. Patents Pubs. 2011/0039654 and 2011/0053730.
One such system, an epicyclic gear system 10, is shown in cross-section in FIG. 14 to include a planet pin 12 about which a sleeve 22 is disposed. A planet gear 26 encompasses the sleeve 22 and is connected thereto through a rib ring 30. The planet pin 12 includes a groove 14 at a central location thereof. The sleeve 22 includes a tapered landing 24. The planet gear 26 has an indent 28. Rollers 32 are positioned between races found on an inner surface of the planet gear 26 and races found on an outer surface of the sleeve 22.
The planet pin 12 is press fit to an upwind carrier plate (not shown). The gear system 10 acts as a double joint system that allows the planet gear 26 to align to a ring gear and a sun gear (not shown) despite the tangential location and misalignment of planet pin 12. There is a gap 24 between the sleeve 22 and the planet pin 12 to allow relative motion and adjustment of the planet pin 12 for rotational and tangential dislocation due to forces being exerted on the planet gear 26.
Planetary gear systems, such as system 10, find use in applications such as wind turbines. Other potential applications can be found in mill operations, the oil and gas industry, and the aviation industry.
For known planetary gear systems utilizing multiple planet gears, a problem that has developed is the creation of an unbalanced load between the planet gears. As a rotating member—depending upon the gear system, the ring gear, the carrier, or the sun gear may supply an input to the gearbox—rotates, it places a force, or a load, on the planet gears. The load factor Kγ of a planetary gear system may be defined as:Kγ=TBranchNCP/TNom Where TBranch is the torque for the gear with the heaviest load, NCP is the number of planets, and TNom is the total nominal torque for the system. Ideally, the force should be the same on each planet gear, i.e., Kγ=1.0, thereby creating a balanced load. However, for a variety of reasons planetary gear systems often suffer from unbalanced loads.
One reason is that the gear teeth of the planetary gears are manufactured with a normal variance for such teeth. For example, the thickness of the gear teeth may vary to an extent expected of tolerances for gear teeth. Additionally, the pitch—the distance between adjacent gear teeth—also may vary.
Under normal manufacturing practices, the planet pin holes in the carrier will be drilled away from their centric true positions. This is due to manufacturing tolerance limitations, complexity of the machined part, measuring capability, and human error. This scenario causes (1) planet pins to be out of alignment from the central shaft, and (2) each planet gear to carry a load different from what they are designed for. Under normal loading conditions, the carrier may twist slightly. This twist may contribute to the misalignment between the planet gears and the ring gear/sun gear assembly. Depending on the number of planets and their respective tolerances, loads experienced by any single planet can increase dramatically, as much as 2× or more.
Reducing the load factor Kγ on a gear system will allow smaller system components to be utilized or allow greater loads on system components than are currently placed. A more evenly distributed shared load may allow for an increase in the gearbox torque density.
With some of these concerns in mind, a planetary gear system that includes planetary gears that self-align as they mesh with a ring and a centralized sun gear would be welcome in the art.